Conventionally, a method for estimating an SOC using an open circuit voltage (OCV) of a cell has been known as an example of a method for estimating an SOC of an electric storage device. In this method, the OCV of the cell is defined, an SOC corresponding to the defined OCV is determined with reference to a predetermined correlation between OCV and SOC, and the determined SOC is determined as an estimated SOC (see Patent Document 1).    Patent Document 1: JP-A-2007-171205
In some electric storage devices, the correlation between OCV and SOC may not be directly proportional but curved. In such electric storage devices, when SOCs are obtained by the method for estimating an SOC using an OCV, a range of errors in SOC relative to errors in defined OCV varies according to the defined OCV. Especially, if the electric storage devices include an electrode that is made of material having OCV-SOC characteristic in that a slight change region extensively exists, a difference between an SOC obtained by the method of estimating an SOC using an OCV and an actual SOC may become large. The slight change region is a region in which a variation in OCV relative to an SOC is small.
An objective of the present invention is to provide a technology for suppressing determination of an SOC obtained by the method of estimating an SOC using an OCV as an estimated SOC even though the obtained SOC is significantly different from an actual SOC.